Document the contents of $datadir/keys

  o Documentation:

    - Document the contents of the 'datadir/keys' subdirectory in the manual

      page. Closes ticekt 17621.

__DataDirectory__**/keys/***::

    Only used by servers. Holds identity keys and onion keys.

__DataDirectory__**/keys/authority_identity_key**::

    A directory authority's master identity key, used to authenticate its

    signing key. Tor doesn't use this while it's running. The tor-gencert

    program uses this. If you're running an authority, you should keep this

    key offline, and not actually put it here.

__DataDirectory__**/keys/authority_certificate**::

    A directory authority's certificate, which authenticates the authority's

    current vote- and consensus-signing key using its master identity key.

    Only directory authorities use this file.

__DataDirectory__**/keys/authority_signing_key**::

    A directory authority's signing key, used to sign votes and consensuses.

    Only directory authorities use this file.  Corresponds to the

    **authority_certificate** cert.

__DataDirectory__**/keys/legacy_certificate**::

    As authority_certificate: used only when V3AuthUseLegacyKey is set.

    See documentation for V3AuthUseLegacyKey.

__DataDirectory__**/keys/legacy_signing_key**::

    As authority_signing_key: used only when V3AuthUseLegacyKey is set.

    See documentation for V3AuthUseLegacyKey.

__DataDirectory__**/keys/secret_id_key**::

    A relay's RSA1024 permanent identity key, including private and public

    components.  Used to sign router descriptors, and to sign other keys.

__DataDirectory__**/keys/ed25519_master_id_public_key**::

    The public part of a relay's Ed25519 permanent identity key.

__DataDirectory__**/keys/ed25519_master_id_secret_key**::

    The private part of a relay's Ed25519 permanent identity key.  This key

    is used to sign the medium-term ed25519 signing key.  This file can be

    kept offline, or kept encrypted. If so, Tor will not be able to generate

    new signing keys itself; you'll need to use tor --keygen yourself to do

    so.

__DataDirectory__**/keys/ed25519_signing_secret_key**::

    The private and public components of a relay's medium-term Ed25519 signing

    key. This key is authenticated by the Ed25519 master key, in turn

    authenticates other keys (and router descriptors).

__DataDirectory__**/keys/ed25519_signing_cert**::

    The certificate which authenticates "ed25519_signing_secret_key" as

    having been signed by the Ed25519 master key.

__DataDirectory__**/keys/secret_onion_key**::

    A relay's RSA1024 short-term onion key. Used to decrypt old-style ("TAP")

    circuit extension requests.

__DataDirectory__**/keys/secret_onion_key_ntor**::

    A relay's Curve25519 short-term onion key. Used to handle modern ("ntor")

    circuit extension requests.

__DataDirectory__**/fingerprint**::

    Only used by servers. Holds the fingerprint of the server's identity key.